A wind turbine is an electric power generating device that captures the energy in the wind by using rotating airfoil blades to develop aerodynamic lift, thereby removing momentum from the air flow and converting it into useful mechanical work. Wind turbines can operate as either horizontal or vertical axis devices, although horizontal axis wind turbines are far more common.
The aerodynamic blades are attached to a hub, which is in turn connected to a main shaft. The main shaft is supported by a main shaft bearing housing assembly which rests on a structural frame. The speed of the main shaft is normally too slow for most generating devices, and so must be increased by transmission, although some modern wind turbines use direct drive generators, avoiding the need for transmissions. The output of the transmission is a high speed or drive shaft that turns the generator to produce electric power. The generator is also supported by the structural frame, which attaches to a yaw bearing, which in turn bolts to a tower. The tower rests on a foundation.
The main shaft bearing housing assembly, the transmission, and the structural frame, or sub groupings thereof are combined in some wind turbine embodiments.
Wind turbines are given a power curve by the manufacturer, and this power curve defines the power production of the wind turbine as a function of wind speed. The power curve includes the rated power, which is the maximum operating power of the turbine, and the wind speed at which the rated power is achieved, called the rated wind speed.
The prior art of pitch to feather for power regulation consists of using a control system to dynamically pivot the blades on a set of blade pitch bearings to change the aerodynamic lift and drag of the blade and consequent mechanical torque that gets transmitted to the generator. The control action is normally achieved by the use of a power transducer and a microprocessor that activates a blade pitch actuator or actuators. The pitch control system can use an electromechanical system, a hydraulic system, or any active method to apply the torque necessary to pivot the blades on the pitch bearings. Other types of power measurements and logic controllers are also in use. Common terminology considers the blades to pitch to lower angles for more power and higher loads, and to pitch to higher angles or feather to reduce power and associated loads. The lowest or most energetic pitch angle is normally zero degrees and the highest pitch angle is 90 degrees used in parking the rotor. Operating with a zero degree pitch angle will yield the highest power and the highest loads, while a 90 degree pitch angle will normally result in the lowest structural loads. In active pitch control systems, the generated power can exceed the rated power in short term excursions, but the turbine and the generator are designed to operate at the rated power, and the average power taken over periods longer than about one minute should not exceed the rated power.
In pitch to feather regulation the aerodynamic angle of attack is adjusted to higher angles of attack (toward feather) thereby spilling excess power and excess loads. Pitch to feather control allows the machine to be stopped and started by pure aerodynamic forces, and a physical brake is needed only for emergencies and to lock the rotor for maintenance activities. There are several types of pitch to feather control algorithms in use. The most common type is the Proportional, Integral, Derivative (PID) control method as taught in U.S. Pat. No. 6,856,039 issued to Mikhail et al. in February 2005, incorporated herein by reference. PID algorithms use the power measurement at each time step of the controller to calculate the error between the measured power and the rated power (the proportional difference). The proportional difference together with the integral and derivative differences are used to decide which direction and how much to change the blade pitch angle. A second method calculates an observer wind speed as taught in U.S. Pat. No. 5,155,375 issued to Holley in October 1992, incorporated herein by reference. A third method is a deadband controller as taught in U.S. Pat. No. 4,426,192 issued to Chertok et al. in January 1984, incorporated herein by reference, which monitors the power and commands a pitch drive when the power is outside the deadband: pitch to feather when the power exceeds the upper deadband, and pitch to power when the power falls below the lower deadband.
A significant cause of load excursions in wind turbines is the action of the controller to reduce the blade pitch angle when the power drops. This action will tend to increase the power back to the rated power value. In random winds, negative wind gusts are often followed by positive gusts, in which case if the pitch angle has been reduced, the positive gust combined with the reduction in pitch angle will result in a rapid rise in power. Consequently, pitch to feather wind turbines experience a wide range of loads. The problem is exacerbated in high winds because as the average wind speed increases, the average pitch angle will increase, and at high pitch angles the aerodynamic forces are much more sensitive to changes in the pitch angle and wind speed, so the load excursions are larger. Variable speed wind turbines perform better than constant speed wind turbines in controlling power generation levels in fluctuating wind by allowing the rotor to speed up and slow down with fluctuations in wind while holding generator torque constant. However, blade loads are not controlled by variable speed wind turbines. Indeed, as the rotor speed increases during a wind gust, the out of plane blade loads, or thrust loads on the rotor, actually increase more than they would if the rotor speed were held constant.
Power regulation limits the average power of a wind turbine to rated power but short term excursions of power can be very large, occasionally going more than twice the rated power. Generators absorb these power excursions at the cost of generator life. Moreover, the aerodynamic forces are not limited to generating productive mechanical torque. The forces on the airfoil that are in the plane of the rotor's rotation are predominantly productive, but the out of plane forces on the airfoil result in predominantly non-productive or parasitic loads and have higher amplitudes than the productive loads. The out of plane forces are perpendicular to the plane of rotation and result in thrust forces on the main shaft and on bending moments at the blade roots and the blade/hub connection. The main shaft will also experience bending moments due to gyroscopic forces when the rotor changes direction, and due to differences in thrust forces between different blades. Pitch control systems generally control the power generating loads, but the larger parasitic loads are only regulated indirectly and also experience large excursions. This makes it necessary to design the turbines heavier to withstand excess loading and forces wind turbine to shut down at wind speeds that are lower than is typically desired. One approach to limit damage from excessive loads due to mechanical vibrations is taught by U.S. Pat. No. 6,525,519 issued to Garnaes in February 2003, incorporated herein by reference. This approach uses spectral analysis to detect excess vibration, in which case the blades are driven to feather until the vibration stops and active control is resumed. Although this will reduce vibration induced loads and associated damage, normal operating parasitic loads in stall can be very large and will not be detected or attenuated.
Another approach to limiting loads is taught in U.S. Pat. No. 6,361,275 issued to Wobben in March 2002, incorporated herein by reference. This method measures loads with strain gages, blade position, and wind speed at the blades and uses the measurement to make instantaneous adjustments to individual blades, a plurality of blades, or all the blades to limit loads. The problem with the Wobben method is that strain gages are notoriously failure prone, getting information from the blades to the controller is failure prone and expensive, and using all this information reliably in real-time is a significant engineering challenge. Moreover, relying on instantaneous blade pitch angle adjustments based on loads will result in the same problem discussed above in which negative and positive gusts combine with pitch changes to produce large load excursions.
Weitkamp teaches in U.S. Patent Application 2002/0000723 A1 published in January of 2002, incorporated herein by reference, a method which uses load measurements to develop an actual load distribution for comparison with a predetermined load distribution and the Cost of Energy to optimize the performance by adjusting the power curve. Simply adjusting the power curve will not limit load excursions. Moreover, the method for optimizing the performance is complex and assumes that the measured load distribution reflects fatigue damage, and that the results can be used to operate close to the margins. These assumptions lower the probability of successful application of the method in limiting fatigue damage without significantly impacting the cost of energy.
Still another approach to limiting loads is taught in U.S. Pat. No. 6,619,918 issued to Rebsdorf in September 2003, incorporated herein by reference, in which blade measurements are used to limit loads, but no method for limiting the loads is given and as above, using instantaneous control may result in large load excursions. Moreover, getting blade measurements to the controller is expensive and failure prone.